Contact printing using high volume presses is commonly employed to print a large number of copies of an image. A contact printing press typically utilizes a printing plate to apply a colorant to a surface to form an image thereon. The surface can form part of a receiver media (e.g. paper) or can form part of an intermediate component adapted to transfer the colorant from its surface to the receiver media (e.g. a blanket cylinder of a press). In either case, a colorant pattern is transferred to the receiver media to form an image on the receiver medium.
Printing plates typically undergo various processes to render them in a suitable configuration for use in a printing press. For example, exposure processes are used to form images on an imageable surface of a printing plate that has been suitably treated so as to be sensitive to light or heat radiation. One type of exposure process employs masks. The masks are typically formed by exposing highly sensitive film media using a laser printer known as an “image-setter.” The film media can be additionally developed to form the mask. The mask is placed in area contact with a sensitized printing plate, which is in turn exposed through the mask. Printing plates exposed in this manner are typically referred to as “conventional printing plates.” Some conventional lithographic printing plates are sensitive to radiation in the ultraviolet region of the light spectrum.
Another conventional method directly forms images on printing plates through the use of a specialized imaging apparatus typically referred to as a plate-setter. A plate-setter in combination with a controller that receives and conditions image data for use by the plate-setter is commonly known as a “computer-to-plate” or “CTP” system. CTP systems offer a substantial advantage over image-setters in that they eliminate film masks and associated process variations associated therewith. Printing plates imaged by CTP systems are typically referred to as “digital” printing plates. Digital printing plates can include photopolymer coatings (i.e. visible light plates) or thermo-sensitive coatings (i.e. thermal plates).
In many printing processes, a plurality of printing plates is used to apply different colorants to a receiver media. Typically, each printing plate applies a different colorant to the receiver media. In this way, the printed image formed on the receiver media can contain different colors. Each of the printing plates must be registered with respect to one another to form a printed image having a desired visual quality. Regardless of the manner by which an image is formed on a printing plate, it must be accurately positioned on the printing plate to achieve a desired registration with the images formed on other associated printing plates.
In some cases, registration features are formed in a printing plate to help register the printing plate on a printing press. The registration features can be formed by various processes including processes adapted to form perforations in the printing plate. A set of perforations can be used to define registration features comprising locating holes or locating channels adapted for providing a desired alignment with a corresponding set of registration features on printing press. It is noted that accurate registration requires that the registration features formed on a printing plate also be registered with the images formed on the printing plate. In some cases, the image forming process and the registration feature forming process are conducted by different apparatus. In other cases, the image forming process and the registration feature forming process are conduced by the same apparatus. In some cases, the image forming process precedes the registration feature forming process while in other cases, the opposite occurs. In some cases, a registration feature formed on a printing plate is employed to assist in the accurate placement of an image on the printing plate. In other cases, an image formed on a printing plate is employed to assist in the accurate placement of a registration feature on the printing plate.
In many cases, one or more edges of a printing plate are used for registration purposes during a processing of the printing plate. For example, during some processes, a printing plate is aligned on a support surface of an apparatus by bringing one or more of the plate edges known as “registration edges” into contact with various registration members. Various groupings of registration members are often employed to register printing plates to the support surface. “Three-point” registration is especially advantageous for rectangular or square shaped printing plates. Once a required contact is established between the printing plate and the registration members, the printing plate is deemed to be in a required registration for a subsequent processing such as the forming of an image or registration feature. Failure to establish the necessary contact between the printing plate and the registration members can introduce registration errors during the subsequent processing. The failure to establish the necessary contact between the printing plate and the registration members is referred to as “misregistration.” Registration errors can lead to reduced quality in the finished printing plate and adversely impact the productivity of the plate making process.
Various conventional printing plate registration detection systems are known. For example, in commonly-assigned U.S. Pat. No. 6,510,793 (Kerr et al.), which is herein incorporated by reference, describe a electronic printing plate registration system in which registration is established when the edges of a printing plate contacts all of three electrically conductive members to create a short between the all of the three conductive members. In one embodiment, Kerr et al. teaches the use of a signal generator that generates an electrical signal at each of two of the three conductive members which act as “emitter” members. An electrical short detection system employs a short detector that senses both the electrical signals at the remaining third conductive member which acts a “receiver” member. In this regard, the electrical detector is adapted to detect both the electrical signals provided by the two “emitter” conductive members. Kerr et al. teaches the use of two signals having different characteristics (e.g. frequency) to determine whether a misregistration is created by an absence of contact between the printing plate and a particular one of the two emitter members. Although this electronic printing plate registration system can identify some misregistrations, it cannot identify others. In particular, this electronic printing plate registration system cannot distinguish between a first misregistration caused by an absence of contact between the printing plate and each of the two emitter members and a second misregistration caused by an absence of contact between the printing plate and the receiver member. In this regard, this uncertainty may hinder corrective actions in an automated system used to correct any misregistration.
There is a need for improved methods and apparatus for properly registering a printing plate during a printing plate processing operation.
There is a need for improved methods and apparatus for correcting a misregistration of a printing plate during a printing plate processing operation.
There is a need for improved methods and apparatus for accurately identifying which of a plurality of different printing plate misregistrations exist.
There is a need for automated methods for correcting an identified misregistration of printing plate during a printing plate processing operation.
There is a need for an imaging apparatus with improved printing plate registration abilities.
There is a need for a perforation apparatus with improved printing plate registration abilities.